Transmit Optical Sub-Assembly (TOSA) components generally consist of optical isolators, monitoring photodiodes, LD driver circuits, thermistors, thermoelectric coolers, automatic temperature control circuits (ATC), and automatic power control circuits (APT). Optical modules are devices used to connect network devices, transmit and receive data between network devices, and can be used to convert optical and electrical signals. The optical module is a very important component in an optical communication system. TOSA is short for Transmitter Optical Sub Assembly. Prior to applying any biasing to a pn junction the concentration of holes (denoted byð¯) is on the p side, while that of electrons is (denoted by r) is on the.